The self deceiving assumption of effective human multitasking.
People are often telling me they are good at multitasking, i.e. handling multiple things at once and performing well at doing so. Now, the human brain can only make a single conscious decision at a time. To understand this, we need to consider that making a conscious decision requires attention, and the very concept of attention means activating relevant information contexts for an observation or decision making and inhibiting other irrelevant information.
The suppression involved in attention control makes it harder for us to continue with a previously executed task, this is why interruptions affect our work flows badly, such as an incoming call, SMS or a door bell. Even just making a decision on whether to take a call already requires attention diversion.
Related, processing emails or surfing while talking to someone on the phone results in bad performance on both tasks, because the attention required for each, necessarily suppresses resources needed by the second task. Now some actions don’t suffer from this competition, we can walk and breathe or balance ourselves fine while paying full attention to a conversation. That’s because we have learned early on in our lives to automate these seemingly mundane tasks, so they don’t require our conscious attention at this point.
Studies   have shown time and again, that working on a single task in isolation yields vastly better results and in a shorter time frame when frequent context switches are avoided. This can be further optimized by training in concentration techniques, such as breath meditation, autogenic training or muscle relaxation.
Here’s a number of tips that will help to put these findings to practical use:
Let go of the idea of permanent reachability, nothing is so urgent that it cannot wait the extra hour to be handled efficiently.
Make up your own mind about when to process emails, SMS, IM, news, voice messages.
Start growing a habit of processing things in batches, e.g. walk through a list of needed phone calls in succession, compose related replies in batches, first queue and later process multiple pending reviews at once, queue research tasks and walk through them in a separate browsing session, etc.
Enforce non-availability periods where you cannot be interrupted and may concentrate on tasks of your choice for an extended period.
Schedule phone meetings in advance, ensure everyone has an agenda at hand for the meeting to avoid distractions (Don’t Call Me).